Ok so bottom line, based on the diagram, you want to find when - that's your question
So at 6pm, 6 hrs have passed; therefore, ship A has travelled 150km south (y1) and ship B has travelled 90 km north (y2). y= y1+y2=240km
x = 70km = CONSTANT
AT THE INSTANT of 6pm, z would be km
Therefore, to solve this question, you use a modified version of pythagorean's theorem:
Note: and are variables, NOT constants.
Plug in all your numbers and solve... should get km/h